1. Field of the Invention
The present invention generally relates to an exposure method, and more particularly to an exposure method used to exposure a substrate to be processed, such as a single crystalline substrate for a semiconductor wafer. The present invention is suitable to, for example, an exposure method of exposing a single crystalline substrate for a semiconductor wafer by a step-and-scan method in a photolithographic process.
2. Related Background Art
Up to now, there has been used a projection exposure apparatus for projecting a circuit pattern drawn on a reticle or a mask (these terms are interchangeably used in the present application) to a wafer or the like by a projection optical system to transfer the circuit pattern when a minute semiconductor device such as a semiconductor memory or a logic circuit is produced using a photolithographic (printing) technique. Among the projection exposure apparatuses, a scanning exposure apparatus that exposes the entire reticle pattern to each region to be exposed of the wafer by scanning the reticle and the wafer in synchronous with each other while a portion of the reticle is illuminated (which is also called a “scanner”) in order to improve resolution and to expand an exposure region, recently attracts lots of attention. The scanning exposure apparatus typically includes a reticle stage and a wafer stage, which are used for scanning the reticle and the wafer between which the projection optical system is interposed.
In the scanning exposure apparatus, a focus detection system is constructed as a focal position detecting unit that measures a displacement in position of the wafer in the optical axis direction of the projection optical system (that is, a displacement between the focal plane of the projection optical system and the wafer surface).
However, when the projection optical system absorbs exposure heat or when a surrounding environment varies, an error is caused between a measurement origin of the focus detection system and the focal plane of the projection optical system. Therefore, in order to measure the error for the correction, a through-the-reticle (TTR) alignment optical system is constructed.
Also, the TTR alignment optical system is generally composed of two optical systems. Therefore, it is possible to simultaneously perform focal measurement at two points. FIG. 13 is a schematic view showing a drive area of a conventional TTR alignment optical system. As shown in FIG. 13, on the proviso that a scanning direction is set to a Y-axis direction, the TTR alignment optical system is constructed such that the first optical system and the second optical system can be driven within drive areas MEa and MEb on an X-axis with a Y-axis set as a symmetry axis in a slit-shaped exposure slit ES in which a direction perpendicular to the scanning direction is a long side and the scanning direction is a short side.
The reason why the first optical system and the second optical system are disposed symmetrically about the Y-axis on the X-axis is to measure a tilt of an image plane in the X-axis direction. Even in the case where the focal measurement is performed on only one point by the TTR alignment optical system and the focus calibration with the focus detection system is performed, when an actual image plane of the projection optical system and an actual exposure surface (printing surface) are tilted, a preferable resolution performance can be obtained on the entire exposure slit ES. In particular, in the case of the scanning exposure apparatus, the exposure region at rest is in a slit shape. Therefore, when the image plane of the projection optical system and the actual exposure surface (that is, the wafer surface) in a direction perpendicular to the scanning direction (long side direction) are tilted, the resolution is lowered.
Thus, for example, as shown in FIG. 13, a focusing condition of the projection optical system is measured at each of a plurality of measurement points KP in the exposure slit ES. Then, the tilt of the image plane in the X-axis direction is obtained and the actual exposure surface is aligned with the image plane based on the obtained tilt, with the result that the preferable resolution performance cannot be obtained.
A reduction in size of a pattern to be transferred, that is, an increase in resolution is required according to an increase in scale of integration of the semiconductor devices. In order to satisfy such requirements, only a reduction in exposure wavelength has been performed up to now. However, the scale of integration of the semiconductor devices is rapidly increasing and it is difficult to cope with this only through the reduction in exposure wavelength. Therefore, in recent years, in order to satisfy the requirement for the increase in resolution, in addition to the reduction in exposure wavelength, a numerical aperture (NA) of the projection optical system is increased from a conventional NA of about 0.6 to a high NA which exceeds 0.8.
Thus, the focal depth becomes extremely smaller than conventional ones. In the exposure apparatus, a significant improvement in detection precision of the focal position, in particular, an improvement in precision with respect to the focus calibration is required. In particular, because of a decrease in focal depth, the following are required as indispensable operations. That is, the tilt of the image plane in the scanning direction, in which a problem is not caused up to now is measured. Then, for example, the actual exposure surface is aligned with the image plane of the projection optical system by driving the wafer stage. Alternatively, the image plane is corrected by driving a lens and the like in the projection optical system, so that it is aligned with the actual exposure surface.
As shown in FIG. 13, in the conventional scanning exposure apparatus, the first optical system and the second optical system which compose the TTR alignment optical system are provided in the exposure slit ES. The focal measurement is performed on the heights of two images in the exposure slit ES symmetrical about the Y-axis. Therefore, according to the conventional scanning exposure apparatus, the tilt of the image plane in the direction perpendicular to the scanning direction can be measured and corrected. However, the tilt of the image plane in the scanning direction cannot be measured.